Currently, for the purpose of cooling electronic devices which contain heating components, such as CPUs and LSIs, piezoelectric actuators which utilize piezoelectric ceramics having the property of extending and contracting by the application of a voltage are extensively used. For example, Patent Document 1 discloses a piezoelectric fan 9 having a sectional configuration shown in FIG. 1.
FIG. 1 is a sectional view illustrating the configuration of the piezoelectric fan 9 disclosed in Patent Document 1. The piezoelectric fan 9 includes a vibration sheet 1, piezoelectric elements 2, a support member 3, and a drive circuit 20.
As shown in FIG. 1, one piezoelectric element 2 is attached to one principal surface of the vibration sheet 1 and the other piezoelectric element 2 is attached to the other principal surface of the vibration sheet 1 with bonding layers 5 therebetween. The support member 3 is attached to one side of the vibration sheet 1 and fixes the vibration sheet 1 at one end in a cantilever structure. The end of the vibration sheet 1 fixed by the support member 3 is a fixed end, and the other end of the vibration sheet 1 is a free end.
The piezoelectric elements 2 and the vibration sheet 1 form a bimorph vibrator in which the two piezoelectric elements 2 sandwich the vibration sheet 1, which serves as an intermediate electrode, from both the principal surfaces. The structure of each of the two piezoelectric elements 2 is such that electrodes 4 are formed on the front and back sides of piezoelectric ceramics 8. The drive circuit 20 which outputs an AC voltage is connected to each of the electrodes 4 and the vibration sheet 1. The piezoelectric elements 2 have been subjected to poling treatment so that, when an AC voltage is applied between each of the electrodes 4 and the vibration sheet 1, the vibration sheet 1 may bend in its thickness direction so as to perform bending vibration.
With the above-described configuration, by the application of an AC voltage between each of the electrode films 4 and the vibration sheet 1 from the drive circuit 20, when one of the piezoelectric elements 2 extends, the other piezoelectric element 2 contracts, and conversely, when one of the piezoelectric elements 2 contracts, the other piezoelectric element 2 extends. In accordance with this extending and contracting operation of the piezoelectric elements 2, the vibration sheet 1 performs bending vibration.
Accordingly, by installing the piezoelectric fan 9 near a heating component, such as an LSI or a CPU, within an electronic device, when an AC voltage is applied to the piezoelectric fan 9, the free end of the vibration sheet 1 swings like a fan. In this manner, cooling air is generated so as to cool the heating component.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2008-172105
However, in the piezoelectric fan 9, due to the bending vibration of the vibration sheet 1, tensile stress is applied to the piezoelectric elements 2, which is in contact with the vibration sheet 1. Although the piezoelectric ceramics 8 of the piezoelectric elements 2 are resistant to compressive stress, they are vulnerable to tensile stress. Accordingly, while the vibration sheet 1 is performing bending vibration, a load is imposed on the piezoelectric ceramics 8 due to the tensile stress applied to the piezoelectric elements 2.
Thus, if the bending vibration of the vibration sheet 1 is performed for a long time and the load accumulates within the piezoelectric ceramics 8, the piezoelectric ceramics 8 crack and are broken.